Fix spelling error in FilesApp test

Change-Id: Iea3c4f398da33308841d04ed95371d2f09a8e38e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2519548
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Auto-Submit: Joel Hockey <joelhockey@chromium.org>
Reviewed-by: Jason Lin <lxj@google.com>
Cr-Commit-Position: refs/heads/master@{#825226}

ui/file_manager/file_manager/background/js/mock_volume_manager.js

@@ -67,7 +67,7 @@
         VolumeManagerCommon.RootType.DRIVE, str('DRIVE_DIRECTORY_LABEL'));
     /** @type {MockFileSystem} */ (drive.fileSystem)
         .populate(Object.values(driveFs.entries));
-    window.webkitResolveLocalFileSystmeURL = orig;
+    window.webkitResolveLocalFileSystemURL = orig;
 
     // Create Downloads.
     this.createVolumeInfo(